[mobile] Save exported project to storage

I want to have a method that can either:

open a file dialog to save to save the export to whatever location or
save the file in the provided storage for the app in a subdirectory with the project title

Current development branch is 1.1.

It might be helpful to isolate any platform specific methods to a single service file. Right now I have a few of them in here that can be migrated in such service as well. Something like src/interface.js, src/bridge.jsor what ever can be created.

The save file dialog will be called here: https://github.com/TorstenDittmann/OmniaWrite/blob/1.1/src/routes/Export/Cloud.svelte#L124

TorstenDittmann / OmniaWrite

[mobile] Save exported project to storage #21